Which of the following can provide power gain?

A transistor is an active electronic component capable of amplifying a signal, meaning it can provide true power gain. It achieves this by using a small input signal to control a larger amount of power drawn from a separate DC power supply. The other options are all passive components, which cannot add power to a circuit. A resistor simply opposes current flow and dissipates power as heat. A reactor (inductor or capacitor) stores energy but does not amplify it. A transformer is a very common distractor here. While a transformer can step up voltage, it simultaneously steps down current, meaning the total output power is never greater than the input power. Only an active component like a transistor can increase total signal power.